The invention relates to a capacitor dielectric with internal barrier layers consisting of a polycrystalline ceramic body made of material of perovskite structure on the basis of barium titanate having the general formula EQU (Ba.sub.1-x M.sub.x.sup.II)O.multidot.z(Ti.sub.1-y M.sub.y.sup.IV)O.sub.2 ;
in which M.sup.II =Ca, Sr, Pb and/or Mg and M.sup.IV =Zr and/or Sn and z assumes the values 1.005 to 1.05, and containing at least two different doping substances of which one (in particular antimony, niobium, lanthanum or bismuth) predominantly causes n conduction inside the crystallites and the other (in particular copper, cobalt, nickel, iron or manganese) predominantly causes p conduction in the surface layer of the crystallites, the proportion of the doping substance causing the n conduction being 1.5 to 2.5 times greater than the maximum doping quantity and the proportion of the substance causing the p conduction being 0.01 to 0.15% by weight.
In addition the invention relates to a method for producing such a capacitor dielectric.
Such a capacitor dielectric is described in German Published specification No. 1,614,605, in the corresponding British Pat. No. 1,204,436, and in U.S. Pat. No. 3,569,802.
A capacitor dielectric disclosed in these specifications has been on the market for several years under the name SIBATIT 50.000 (SIBATIT=registered trade mark) and has undergone considerable technical testing. This capacitor dielectric can be used in the form of disks, tubes of circular and of rectangular cross-section, always provided with metal layers (e.g. silver or nickel) as electrodes, and also in the form of so-called "stack capacitors". Stack capacitors are capacitors in which thin layers of dielectric material are disposed one over another alternately with metal layers projecting to the edge on different sides in alternating fashion.
In order to bring about maximum conductivity inside the core with the p doping substance present at the same time despite the proportions of n doping substance which are higher than those normally needed for maximum conductivity (maximum doping), the named specifications propose by way of a preferred production method that all substances be made to react together in oxide form, for in this case the conductivity inside the crystallite cores reaches optimum values while the p doping substance which can only be integrated into the perovskite crystal lattice to a limited degree, if at all, particularly the copper, is essentially incorporated in the surface layer of the crystallites.
When one speaks of values for the dielectric constant (DK) in the case of a capacitor dielectric with internal barrier layers, one is always referring to apparent DK values here, since it is assumed when establishing the DK from measurement of the capacitance of a capacitor of this kind that the entire body has a high .epsilon. whereas in fact only the very thin p-n junctions at the grain boundaries become dielectrically effective (barrier layers), exhibiting a normal DK value for barium titanate but giving a DK increased by a multiple because it is related to the entire body.
With a capacitor dielectric, the DK is not alone in playing a part with respect to the level of capacitance for it is also necessary that the dependence of the DK upon an operating temperature, the tangent of the loss angle (loss factor) and the insulation and thus the load capacity of the capacitor fall within certain limits.
This is already largely the case with the capacitor dielectrics with internal barrier layers indicated as being known at the start. However, efforts are being made to improve the electrical properties and to make it even easier to reproduce the electrical values in mass production.
As a criterion for the improvement in the electrical properties of electrical capacitors use is made of the time constant, i.e. the produce of the capacitance of the capacitor and its insulation resistance, indicated in M.OMEGA..multidot..mu. F or, after conversion, indicated in seconds.
The time constant should always be indicated in conjunction with the field strength at which measurement is made, since both the capacitance and the insulating performance depend upon the prevailing field strength.
The known barrier layer capacitors with internal barrier layers have time constants of between 20 and 70 seconds with a field strength of 100 V/mm or between 2 and 20 seconds with a field strength of 200 V/mm.
In the present connection, improvement in the reproducibility of the electrical values in mass production means that the electrical values to be obtained with less deviation when bodies of the same initial composition and having undergone the same preliminary treatment are sintered in different furnaces, for example.
German patent specification No. 909,819, which essentially corresponds to U.S. Pat. No 2,402,515 as far as the present matter is concerned, describes materials of high dielectric constant and methods for their production according to which, among other things, prefabricated titanates and other perovskite materials are pressed into the form of bodies after grinding and mixing and then sintered. Here barium titanate is mixed with strontium or calcium titanate or barium titanate is mixed with strontium stannate.
These known ceramic dielectrics exhibit no internal barrier layers but are homogeneous dielectric materials with high insulating performance and dielectric constants of between 1000 and 9000 in the case of the barium titanate series. The purpose of mixing pre-baked titanates in the above patents is that the dielectric body have an .epsilon. value dependent as little as possible upon the temperature between 20.degree. and 150.degree. C., for example. In pure form, barium titanate or other perovskite materials have a relatively low DK which increases dramatically in the area of the Curie temperature and then falls again very sharply above the Curie temperature. The mixing of pre-baked titanates is intended to combine several such peak values together so that the resultant .epsilon. temperature curve is flat with relatively high .epsilon. values.
Information on the loss factor with the known capacitor dielectrics shows that relatively good values are obtained.
In the case of capacitors with internal barrier layers in which the inside of the crystallite grains is highly conductive, the loss factor is inevitably considerably higher.
The high conductivity inside the grains also produces comparatively poor insulation resistance values.
The reproducibility of the electrical values also poses considerable problems in capacitor dielectrics with internal barrier layers because ultimately the electrical values are greatly dependent on the individual treatment stages.
To get around all these difficulties, with capacitor dielectrics having internal barrier layers the production method up to now (very tight tolerance temperature-time control of the baking process) has been directed so that the crystallites remain as small as possible in the body, i.e. in practice in the range from 20 to 300 .mu.m, and mixed titanates made together from the oxides have been used as the starting point, and not titanate mixtures.
German Pat. No. 1,490,659, which essentially corresponds to U.S. Pat. No. 3,637,532, describes a sintered electrical cold conductor resistor and a method for its production in which titanates pre-fabricated per se are mixed with one another in order to increase the load quotients, i.e. the ratio of maximum resistance at 200 V/cm to maximum resistance at 10 V/cm -- expressed as a percentage.
Admittedly sintered electrical cold conductor resistors of the known kind are also made of barium titanate and substituted barium titanate and are also doped with some considerably different amounts of antimony and copper here, but with these known bodies there is no intention of forming internal barrier layers in such a way that the material can be used as a capacitor dielectric with internal depletion layers. Thus the properties found with sintered electrical cold-conductor resistors cannot be transferred to dielectric bodies with internal barrier layers.
Here in fact the object is to bring the tolerance limits for the electrical values, some of which are relatively broad, closer together, i.e. to make the tolerances for the electrical values arising from production conditions tighter and thus make the desired electrical values in question even easier to reproduce accurately in mass production. Excessively broad limits for the electrical values are obtained in particular when rutile is used as the TiO.sub.2 component for the production of the perovskite forming material because then the DK values becomes relatively high (up to 100,000) on the one hand and on the other the scattering of these DK values from capacitor to capacitor within the same baking batch becomes comparatively large with different sintering conditions (e.g. different sintering furnaces). In addition the insulation resistance deteriorates substantially.